The Integumentary System: Structure and Function

The Integumentary System

Overview and Components

The integumentary system is the body's largest organ system, consisting of the skin and its accessory structures such as hair, glands, and nails. It serves as a protective barrier, regulates temperature, and provides sensory information.

• Skin: Composed of multiple tissue types, including epithelial, connective, muscle, and nervous tissue.

• Accessory Structures: Hair, glands (sebaceous and sweat), and nails.

Skin Layers

The skin is composed of two main layers: the epidermis and the dermis. Beneath the dermis lies the hypodermis (subcutaneous tissue), which connects the skin to underlying tissues but is not considered part of the integument.

• Epidermis: Outermost layer, composed of stratified squamous keratinized epithelium.

• Dermis: Deeper layer, composed of connective tissue, blood vessels, and sensory receptors.

• Hypodermis: Subcutaneous layer containing adipose tissue for insulation and energy storage.

The Epidermis

Layers of the Epidermis

The epidermis consists of several layers (strata) of cells, each with distinct functions. From superficial to deep, the layers are:

• Stratum Corneum: Superficial layer of dead, keratinized cells (keratinocytes) that provide a tough, protective barrier.

• Stratum Lucidum: Thin, translucent layer of dead cells found only in thick skin (palms and soles).

• Stratum Granulosum: Contains keratinocytes with granules that release waterproofing substances.

• Stratum Spinosum: First layer with metabolically active cells; contains melanin for UV protection.

• Stratum Basale: Deepest layer; single row of dividing cells, including melanocytes and Merkel cells.

Specialized Cells of the Epidermis

• Keratinocytes: Main cell type, produce keratin for protection.

• Melanocytes: Produce melanin pigment, found in the stratum basale.

• Merkel Cells: Sensory cells for light touch, also in the stratum basale.

Basement Membrane

The basement membrane is located immediately deep to the stratum basale. It consists of the basal lamina (produced by keratinocytes) and the reticular lamina (produced by the dermis). This structure anchors the epidermis to the dermis and prevents separation.

Mnemonic for Epidermal Layers

To remember the order of the epidermal layers (superficial to deep): Come, Let's Get Sun Burned (Corneum, Lucidum, Granulosum, Spinosum, Basale).

Cell Death in the Epidermis

The epidermis is avascular, so only cells near the dermis (stratum basale, spinosum, and lower granulosum) receive nutrients and oxygen. As cells migrate outward, they die and become keratinized.

The Dermis

Layers of the Dermis

The dermis is a connective tissue layer beneath the epidermis, divided into two regions:

• Papillary Layer: Superficial, composed of loose connective tissue. Contains dermal papillae (fingerlike projections) that form fingerprints and house tactile (Meissner's) corpuscles and capillary loops.

• Reticular Layer: Deeper, thicker layer of dense irregular connective tissue. Contains sweat glands, sebaceous glands, blood vessels, lamellated (Pacinian) corpuscles, and free nerve endings for pain sensation.

Accessory Structures of the Skin

Hair

Hair consists of dead, keratinized cells and is divided into:

• Shaft: Visible part above the skin surface.

• Root: Embedded in the dermis, surrounded by the hair follicle.

• Arrector Pili Muscle: Smooth muscle that causes hair to stand erect when contracted (goosebumps).

Nails

Nails are protective coverings composed of dead, keratinized cells. Main parts include:

• Nail Plate: Main body of the nail.

• Nail Folds: Skin folds on three sides of the nail.

• Nail Matrix: Proximal region where nail growth occurs.

• Lunula: Visible white crescent at the nail base.

• Eponychium: Cuticle, skin covering the nail plate.

• Hyponychium: Skin under the free edge of the nail.

Glands

• Sebaceous Glands: Exocrine glands that secrete sebum (oil) into hair follicles for lubrication.

• Sweat Glands:

  • Apocrine: Found in armpits and groin; secrete fatty sweat that bacteria break down, causing odor.

  • Eccrine (Merocrine): Most common; found all over the body and secrete watery sweat for thermoregulation.

Thin Skin vs. Thick Skin

Comparison of Thin and Thick Skin

Feature	Thick Skin	Thin Skin
Location	Palms, soles	Everywhere else
Layers	5 (includes stratum lucidum)	4 (no stratum lucidum)
Hair/Sebaceous Glands	Absent	Present
Dermis Thickness	Thinner	Thicker
Dermal Papillae	More pronounced	Less pronounced

Sensory Receptors in the Skin

Types of Sensory Receptors

• Tactile (Meissner's) Corpuscles: Detect fine touch, located in dermal papillae near the skin surface.

• Merkel Disks: Specialized cells in the stratum basale for light touch, especially in fingertips.

• Pacinian (Lamellated) Corpuscles: Detect deep pressure and vibration, located in the reticular dermis and hypodermis.

• Free Nerve Endings: Detect pain, temperature, and crude touch.

Summary Table: Structures to Identify in the Skin

Structure	Location/Function
Epidermis	Outer protective layer
Dermis	Connective tissue, supports epidermis
Stratum Corneum	Dead, keratinized cells
Stratum Lucidum	Only in thick skin
Stratum Granulosum	Keratin granules, waterproofing
Stratum Spinosum	Melanin, active cells
Stratum Basale	Dividing cells, melanocytes, Merkel cells
Papillary Layer	Dermal papillae, Meissner's corpuscles
Reticular Layer	Sweat/sebaceous glands, Pacinian corpuscles
Arrector Pili	Hair erection
Hair Follicle/Bulb/Root/Shaft	Hair growth and structure
Eccrine Sweat Gland	Thermoregulation
Apocrine Sweat Gland	Odorous sweat
Sebaceous Gland	Secretes sebum
Meissner's Corpuscle	Fine touch
Pacinian Corpuscle	Pressure/vibration